Systems and methods for fabricating a multilayer optical structure

ABSTRACT

Systems and methods for fabricating optical elements in accordance with various embodiments of the invention are illustrated. One embodiment includes a method for fabricating an optical element, the method including providing a first optical substrate, depositing a first layer of a first optical recording material onto the first optical substrate, applying an optical exposure process to the first layer to form a first optical structure, temporarily erasing the first optical structure, depositing a second layer of a second optical recording material, and applying an optical exposure process to the second layer to form a second optical structure, wherein the optical exposure process includes using at least one light beam traversing the first layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The current application claims the benefit of and priority under 35U.S.C. § 119(e) to U.S. Provisional Patent Application No. 62/703,329entitled “Systems and Methods for Fabricating a Multilayer OpticalStructure,” filed Jul. 25, 2018. The disclosure of U.S. ProvisionalPatent Application No. 62/703,329 is hereby incorporated by reference inits entirety for all purposes.

FIELD OF THE INVENTION

The present disclosure relates to methods for fabricating multilayeroptical devices and, more particularly, to methods for fabricatingmultilayer holographic waveguide devices using a liquid crystal andpolymer material.

BACKGROUND

Waveguides can be referred to as structures with the capability ofconfining and guiding waves (i.e., restricting the spatial region inwhich waves can propagate). One subclass includes optical waveguides,which are structures that can guide electromagnetic waves, typicallythose in the visible spectrum. Waveguide structures can be designed tocontrol the propagation path of waves using a number of differentmechanisms. For example, planar waveguides can be designed to utilizediffraction gratings to diffract and couple incident light into thewaveguide structure such that the in-coupled light can proceed to travelwithin the planar structure via total internal reflection (“TIR”).

Fabrication of waveguides can include the use of material systems thatallow for the recording of holographic optical elements within thewaveguides. One class of such material includes polymer dispersed liquidcrystal (“PDLC”) mixtures, which are mixtures containingphotopolymerizable monomers and liquid crystals. A further subclass ofsuch mixtures includes holographic polymer dispersed liquid crystal(“HPDLC”) mixtures. Holographic optical elements, such as volume phasegratings, can be recorded in such a liquid mixture by illuminating thematerial with two mutually coherent laser beams. During the recordingprocess, the monomers polymerize and the mixture undergoes aphotopolymerization-induced phase separation, creating regions denselypopulated by liquid crystal micro-droplets, interspersed with regions ofclear polymer. The alternating liquid crystal-rich and liquidcrystal-depleted regions form the fringe planes of the grating.

Waveguide optics, such as those described above, can be considered for arange of display and sensor applications. In many applications,waveguides containing one or more grating layers encoding multipleoptical functions can be realized using various waveguide architecturesand material systems, enabling new innovations in near-eye displays foraugmented reality (“AR”) and virtual reality (“VR”), compact heads-updisplays (“HUDs”) for aviation and road transport, and sensors forbiometric and laser radar (“LIDAR”) applications.

SUMMARY OF THE INVENTION

Systems and methods for fabricating optical elements in accordance withvarious embodiments of the invention are illustrated. One embodimentincludes a method for fabricating an optical element, the methodincluding providing a first optical substrate, depositing a first layerof a first optical recording material onto the first optical substrate,applying an optical exposure process to the first layer to form a firstoptical structure, temporarily erasing the first optical structure,depositing a second layer of a second optical recording material, andapplying an optical exposure process to the second layer to form asecond optical structure, wherein the optical exposure process includesusing at least one light beam traversing the first layer.

In another embodiment, the method further includes providing a secondoptical substrate, wherein the second layer is deposited onto the secondoptical substrate, and overlapping the second optical substrate with thefirst optical substrate.

In a further embodiment, the second optical substrate is laterally orrotationally displaced relative to the first optical substrate.

In still another embodiment, the method further includes applying afirst cover layer to the first layer and applying a second cover layerto the second layer.

In a still further embodiment, the at least one light beam is providedby an apparatus selected from the group that includes: a crossed-beamholographic recording apparatus; a contact copying apparatus using amaster grating or hologram; and an apparatus for traversing light with apredefined beam cross section.

In yet another embodiment, the first optical structure is temporarilyerased by applying an external stimulus.

In a yet further embodiment, the external stimulus includes a stimulusselected from the group that includes: an optical stimulus, a thermalstimulus, a chemical stimulus, a mechanical stimulus, an electricalstimulus, and a magnetic stimulus.

In another additional embodiment, the external stimulus is applied at astrength below a predefined threshold to produce optical noise below apredefined level.

In a further additional embodiment, the method further includestemporarily erasing the second optical structure, depositing a thirdlayer of a third optical recording material, applying an opticalexposure process to the third layer to form a third optical structureusing at least one light beam traversing the first layer and the secondlayer.

In another embodiment again, at least one of the first and secondoptical structures modifies at least one of phase, amplitude, andwavefront of incident light.

In a further embodiment again, the first optical recording material andthe second optical recording material include different materialformulations.

In still yet another embodiment, the first optical recording materialincludes a mixture of liquid crystal and polymer and the first opticalstructure includes at least one grating.

In a still yet further embodiment, the first optical recording materialfurther includes at least one of: a LPP, a dye, a photoinitiator, asurfactant, a multi-function monomer, and nanoparticles.

In still another additional embodiment, temporarily erasing the firstoptical structure includes changing the order parameter of the liquidcrystal.

In a still further additional embodiment, the first optical recordingmaterial includes a liquid crystal, polymer, and an additive fortemporarily erasing the first optical structure.

In still another embodiment again, the first optical recording materialis deposited onto the first optical substrate using spin coating orinkjet printing.

In a still further embodiment again, the first optical substrate iscurved.

In yet another additional embodiment, the method further includes atleast one of the steps of: forming an air gap; applying a layer of lowrefractive index material; applying a polarization control layer; andapplying a liquid crystal alignment layer.

In a yet further additional embodiment, the method forms part of aroll-to-roll fabrication process.

A yet another embodiment again includes a method of fabricating anoptical element, the method including providing first and second opticalsubstrates, forming a first cell from the first and second substrates,filling the first cell with a first optical recording material, applyingan optical exposure process to the first cell to form a first opticalstructure, temporarily erasing the first optical structure, providingthird and fourth optical substrates, forming a second cell from thethird and fourth substrates, filling the second cell with a secondoptical recording material, overlapping the first and second cells, andapplying an optical exposure process to the second layer to form asecond optical structure, wherein the optical exposure process includesusing at least one light beam traversing the first layer.

Additional embodiments and features are set forth in part in thedescription that follows, and in part will become apparent to thoseskilled in the art upon examination of the specification or may belearned by the practice of the invention. A further understanding of thenature and advantages of the present invention may be realized byreference to the remaining portions of the specification and thedrawings, which forms a part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The description will be more fully understood with reference to thefollowing figures and data graphs, which are presented as exemplaryembodiments of the invention and should not be construed as a completerecitation of the scope of the invention. It will apparent to thoseskilled in the art that the present invention may be practiced with someor all of the present invention as disclosed in the followingdescription.

FIGS. 1A and 1B conceptually illustrate different views of a waveguideproviding a total internal reflection light guiding structure.

FIG. 2A-2F conceptually illustrate a method for recording a stack of twogratings in accordance with an embodiment of the invention.

FIG. 3A conceptually illustrates an ordered liquid crystal phase.

FIG. 3B conceptually illustrates a disordered liquid crystal phase.

FIG. 4 conceptually illustrates a flow chart of a method for fabricatinga multi-waveguide layer stack in accordance with an embodiment of theinvention.

FIG. 5 conceptually illustrates a flow chart of a method for fabricatinga multi-waveguide layer stack having two grating layers separated by asubstrate in accordance with an embodiment of the invention.

FIG. 6 conceptually illustrates a flow chart of a method for fabricatinga multi-waveguide layer stack having two grating layers eachencapsulated within a cell in accordance with an embodiment of theinvention.

DETAILED DESCRIPTION

Following below are more detailed descriptions of various conceptsrelated to, and embodiments of, optical displays, methods forfabricating optical displays, and methods for displaying information. Itshould be appreciated that various concepts introduced and discussedbelow may be implemented in any of numerous ways, as the disclosedconcepts are not limited to any particular manner of implementation.Examples of specific implementations and applications are providedprimarily for illustrative purposes. A more complete understanding ofthe invention can be obtained by considering the following detaileddescription in conjunction with the accompanying drawings, wherein likeindex numerals indicate like parts. For the purposes of describingembodiments, some well-known features of optical technology known tothose skilled in the art of optical design and visual displays have beenomitted or simplified in order to not obscure the basic principles ofthe invention. Unless otherwise stated, the term “on-axis” in relationto a ray or a beam direction refers to propagation parallel to an axisnormal to the surfaces of the optical components described in relationto the invention. In the following description the terms light, ray,beam, and direction may be used interchangeably and in association witheach other to indicate the direction of propagation of electromagneticradiation along rectilinear trajectories. The term light andillumination may be used in relation to the visible and infrared bandsof the electromagnetic spectrum. Parts of the following description willbe presented using terminology commonly employed by those skilled in theart of optical design. As used herein, the term grating may encompass agrating comprised of a set of gratings in some embodiments. Forillustrative purposes, it is to be understood that the drawings are notdrawn to scale unless stated otherwise.

Waveguides can be stacked to combine different spectral bandwidths,angular bandwidths, and various other optical functions. However,stacking and aligning waveguides can introduce the risk of planarity andcontamination and can increase the overall processing time and costs.One method for fabricating multilayer holographic waveguide devicesincludes stacking waveguide cells and recording holographic elements onthe complete stack. However, in such methods, a recorded grating in onelayer can deflect the beams traversing it to corrupt a second grating.Similar problems can arise in other processes for manufacturing devicescontaining multiple layers supporting optical structures. As such, manysystems and methods in accordance with various embodiments of theinvention are designed to provide optically efficient, economicalsolutions for fabricating multilayer optical devices, such as but notlimited to holographic waveguide devices.

Solutions for fabricating multilayer optical devices in accordance withvarious embodiments of the invention can include a variety of differenttechniques, including but not limited to methods for recording opticalstructures into layers of optical recording material for waveguidestacks of two or more overlapping layers. In many embodiments, thefabrication process includes fabricating a stack of optical structuresin which a first optical recording material layer deposited on asubstrate is exposed to form a first optical structure, which can betemporarily erased so that a second optical structure can be recordedinto a second material layer deposited onto the first layer usingoptical recording beams traversing the first layer. Optical structurescan include but are not limited to gratings. Temporarily “erased”optical structures or gratings can behave similar to transparentmaterials, allowing light to pass through without affecting the raypaths. This principle can be applied to fabricate a variety of differentwaveguide stack configurations. For example, some processes includefabricating a multilayer waveguide stack with two grating layers thatare separated by a substrate. In some embodiments, the two gratinglayers are each covered by a protective cover layer. In severalembodiments, the process includes fabricating a multilayer waveguidestack with two grating layers that are each encapsulated in a cell. In anumber of embodiments, the process is implemented as part of aroll-to-roll fabrication process. These and other configurations andmethods for fabricating such configurations are discussed in thesections below in further detail.

Holographic Waveguide Devices

Holographic waveguide devices and related methods of manufacturing inaccordance with various embodiments of the invention can be configuredin many different ways. In several embodiments, the device includes anoptical waveguide that is formed with a grating layer sandwiched betweentwo transparent substrates. In such configurations, the waveguide canprovide a total internal reflection (“TIR”) light guiding structureusing the substrate-air interfaces. Light traveling within the waveguidevia TIR can be coupled out of the waveguide when a grating within thegrating layer diffracts the light at an angle beyond the TIR condition.An example of a waveguide utilizing TIR to propagate incident light raysis conceptually illustrated in FIGS. 1A and 1B. FIG. 1A conceptuallyillustrates a perspective view of the waveguide 100 having a gratinglayer 102 sandwiched by transparent substrates 104, 106. Various typesof materials can be used to form the grating layer and substrates. Inmany embodiments, the substrates are made of glass or plastic polymerswhile the grating layer is formed from an HPDLC mixture. FIG. 1B showsthe waveguide 100 in operation with a ray 108 traveling between thewaveguide outer surfaces 104A, 106A via TIR. As shown, the grating layer102 is in contact with the substrate surfaces 104B, 106B.

In many display applications, a holographic waveguide device can beimplemented with an optical structure that includes a layer containingone or more volume holograms or gratings. Optical structures inaccordance with various embodiments of the invention can include variousconfigurations of gratings. In many embodiments, the optical structureincludes grating configuration for two-dimensional beam expansion. Forexample, many optical structures include a fold grating for verticalbeam expansion and beam steering and an output grating for horizontalbeam expansion and extraction of light from the waveguide. Some opticalstructures include an input coupler for the waveguide, which can takethe form of an input grating or prism. In several embodiments, the inputcoupler is a surface relief grating. In other embodiments, the inputcoupler is a volume grating.

Optical structures recorded in waveguides can include many differenttypes of optical elements, such as but not limited to diffractiongratings. In many embodiments, the grating implemented is a Bragggrating (also referred to as a volume grating). Bragg gratings can havehigh efficiency with little light being diffracted into higher orders.The relative amount of light in the diffracted and zero order can bevaried by controlling the refractive index modulation of the grating, aproperty that is can be used to make lossy waveguide gratings forextracting light over a large pupil. One class of gratings used inholographic waveguide devices is the Switchable Bragg Grating (“SBG”).SBGs can be fabricated by first placing a thin film of a mixture ofphotopolymerizable monomers and liquid crystal material between glassplates or substrates. In many cases, the glass plates are in a parallelconfiguration. One or both glass plates can support electrodes,typically transparent tin oxide films, for applying an electric fieldacross the film. The grating structure in an SBG can be recorded in theliquid material (often referred to as the syrup) throughphotopolymerization-induced phase separation using interferentialexposure with a spatially periodic intensity modulation. Factors such asbut not limited to control of the irradiation intensity, componentvolume fractions of the materials in the mixture, and exposuretemperature can determine the resulting grating morphology andperformance. As can readily be appreciated, a wide variety of materialsand mixtures can be used depending on the specific requirements of agiven application. In many embodiments, HPDLC material is used. Duringthe recording process, the monomers polymerize and the mixture undergoesa phase separation. The LC molecules aggregate to form discrete orcoalesced droplets that are periodically distributed in polymer networkson the scale of optical wavelengths. The alternating liquid crystal-richand liquid crystal-depleted regions form the fringe planes of thegrating, which can produce Bragg diffraction with a strong opticalpolarization resulting from the orientation ordering of the LC moleculesin the droplets.

The resulting volume phase grating can exhibit very high diffractionefficiency, which can be controlled by the magnitude of the electricfield applied across the film. When an electric field is applied to thegrating via transparent electrodes, the natural orientation of the LCdroplets can change, causing the refractive index modulation of thefringes to lower and the hologram diffraction efficiency to drop to verylow levels. Typically, the electrodes are configured such that theapplied electric field will be perpendicular to the substrates. In anumber of embodiments, the electrodes are fabricated from indium tinoxide (“ITO”). In the OFF state with no electric field applied, theextraordinary axis of the liquid crystals generally aligns normal to thefringes. The grating thus exhibits high refractive index modulation andhigh diffraction efficiency for P-polarized light. When an electricfield is applied to the HPDLC, the grating switches to the ON statewherein the extraordinary axes of the liquid crystal molecules alignparallel to the applied field and hence perpendicular to the substrate.In the ON state, the grating exhibits lower refractive index modulationand lower diffraction efficiency for both S- and P-polarized light.Thus, the grating region no longer diffracts light. Each grating regioncan be divided into a multiplicity of grating elements such as forexample a pixel matrix according to the function of the HPDLC device.Typically, the electrode on one substrate surface is uniform andcontinuous, while electrodes on the opposing substrate surface arepatterned in accordance to the multiplicity of selectively switchablegrating elements.

Typically, the SBG elements are switched clear in 30 μs with a longerrelaxation time to switch ON. Note that the diffraction efficiency ofthe device can be adjusted, by means of the applied voltage, over acontinuous range. In many cases, the device exhibits near 100%efficiency with no voltage applied and essentially zero efficiency witha sufficiently high voltage applied. In certain types of HPDLC devices,magnetic fields can be used to control the LC orientation. In some HPDLCapplications, phase separation of the LC material from the polymer canbe accomplished to such a degree that no discernible droplet structureresults. An SBG can also be used as a passive grating. In this mode, itschief benefit is a uniquely high refractive index modulation. SBGs canbe used to provide transmission or reflection gratings for free spaceapplications. SBGs can be implemented as waveguide devices in which theHPDLC forms either the waveguide core or an evanescently coupled layerin proximity to the waveguide. The glass plates used to form the HPDLCcell provide a total internal reflection (“TIR”) light guidingstructure. Light can be coupled out of the SBG when the switchablegrating diffracts the light at an angle beyond the TIR condition.

One of the known attributes of transmission SBGs is that the LCmolecules tend to align with an average direction normal to the gratingfringe planes (i.e., parallel to the grating or K-vector). The effect ofthe LC molecule alignment is that transmission SBGs efficiently diffractP polarized light (i.e., light with a polarization vector in the planeof incidence), but have nearly zero diffraction efficiency for Spolarized light (i.e., light with the polarization vector normal to theplane of incidence). As a result, transmission SBGs typically cannot beused at near-grazing incidence as the diffraction efficiency of anygrating for P polarization falls to zero when the included angle betweenthe incident and reflected light is small. In addition, illuminationlight with non-matched polarization is not captured efficiently inholographic displays sensitive to one polarization only.

In some cell designs, adhesives and spacers can be disposed between thesubstrates to affix the layers of the elements together and to maintainthe cell gap, or thickness dimension. In these devices, spacers can takemany forms, such as but not limited to different materials, sizes, andgeometries. Materials can include, for example, plastics (e.g.,divinylbenzene), silica, and conductive spacers. They can take anysuitable geometry, such as but not limited to rods and spheres. Thespacers can take any suitable size. In many cases, the sizes of thespacers range from 1 to 30 μm. While the use of these adhesive materialsand spacers can be necessary in LC cells using conventional materialsand methods of manufacture, they can contribute to the haziness of thecells degrading the optical properties and performance of the waveguideand device.

Waveguides and associated optical structures can be fabricated using avariety of different methods. In many embodiments, a waveguide isfabricated by coating a first substrate with an optical recordingmaterial. In a number of embodiments, the optical recording material isdeposited onto the substrates using spin coating or spraying. A secondsubstrate layer can be included to form the waveguide such that theoptical recording material is sandwiched between two substrates. Inseveral embodiments, the second substrate can be a thin protective filmcoated onto the exposed layer. In various embodiments, the substratesare used to make a cell, which is then filled with the holographicrecording material. The filling process can be accomplished using avariety of different methods, such as but not limited vacuum fillingmethods. In further embodiments, alignment layers and/or polarizationlayers can be added. As can readily be appreciated, the fabricationmethods described can be applied to fabricate a wide variety ofwaveguides with different optical structures, such as but not limited todiffraction gratings. For example, fabrication methods in accordancewith various embodiments of the invention can include recording an SBGby coating an optical recording material onto a substrate, which isexposed and then sealed by a protective overcoat layer.

Various recording methods can be used for fabricating optical structuresin accordance with many embodiments of the invention. In massproduction, it can be more efficient and cost effective to replace thetraditional two beam holographic recording processes with one usingcontact printing from a master. In some embodiments, the gratings arerecorded using mastering and contact copying process. In severalembodiments, the grating in a given layer can be recorded in stepwisefashion by scanning or stepping the recording laser beams across thegrating area.

In many applications, a waveguide stack of two or more waveguides isimplemented for various purposes. For example, two or more waveguidescan be stacked to combine different spectral bandwidths, angularbandwidths, and/or optical functions. Such waveguide stacks can beformed with waveguides that are overlaid. In many embodiments, thewaveguides are overlaid in contact. In other embodiments, the waveguidesare overlaid with air gap(s) or other layer(s) in between. Methods formanufacturing multilayer waveguide devices can include the use ofcertain materials that allow for the individual recording of the opticalstructure within each of the layer within the waveguide device. In thecase of holographic waveguide embodiments, the optical recordingmaterial forming the grating layer can include a liquid crystal (“LC”)polymer mixture. Such material systems can allow the grating to betemporarily erased through the application of external stimuli thatalter the alignment of the LC so that the LC index matches that of thesurrounding polymer. Although discussions may describe the recording ofoptical structures having at least one holographic grating formed inlayers in waveguide devices, various embodiments in accordance with theinvention may also be applied to the recording of more general opticalstructures for modifying at least one of phase, amplitude, or wavefrontof incident light in liquid crystal and polymer material systems.Examples of material systems used in the fabrication processes ofvarious optical devices incorporating waveguides with holographicgratings can include PDLC mixtures and formulations. Discussions of PDLCmaterial systems are described in further detail in the sections below.Although the discussions concentrate on LC polymer material systems,various embodiments in accordance with the invention can be appliedusing other material systems capable of supporting optical structuresthat can be erased by an external stimulus.

Optical Recording Material Systems

PDLC mixtures in accordance with various embodiments of the inventiongenerally include LC, monomers, photoinitiator dyes, and coinitiators.The mixture (often referred to as syrup) frequently also includes asurfactant. For the purposes of describing the invention, a surfactantis defined as any chemical agent that lowers the surface tension of thetotal liquid mixture. The use of surfactants in PDLC mixtures is knownand dates back to the earliest investigations of PDLCs. For example, apaper by R. L Sutherland et al., SPIE Vol. 2689, 158-169, 1996, thedisclosure of which is incorporated herein by reference, describes aPDLC mixture including a monomer, photoinitiator, coinitiator, chainextender, and LCs to which a surfactant can be added. Surfactants arealso mentioned in a paper by Natarajan et al, Journal of NonlinearOptical Physics and Materials, Vol. 5 No. I 89-98, 1996, the disclosureof which is incorporated herein by reference. Furthermore, U.S. Pat. No.7,018,563 by Sutherland; et al., discusses polymer-dispersed liquidcrystal material for forming a polymer-dispersed liquid crystal opticalelement comprising: at least one acrylic acid monomer; at least one typeof liquid crystal material; a photoinitiator dye; a coinitiator; and asurfactant. The disclosure of U.S. Pat. No. 7,018,563 is herebyincorporated by reference in its entirety.

The patent and scientific literature contains many examples of materialsystems and processes that can be used to fabricate SBGs, includinginvestigations into formulating such material systems for achieving highdiffraction efficiency, fast response time, low drive voltage, and soforth. U.S. Pat. No. 5,942,157 by Sutherland, and U.S. Pat. No.5,751,452 by Tanaka et al. both describe monomer and liquid crystalmaterial combinations suitable for fabricating SBG devices. Examples ofrecipes can also be found in papers dating back to the early 1990s. Manyof these materials use acrylate monomers, including:

-   -   R. L. Sutherland et al., Chem. Mater. 5, 1533 (1993), the        disclosure of which is incorporated herein by reference,        describes the use of acrylate polymers and surfactants.        Specifically, the recipe comprises a crosslinking        multifunctional acrylate monomer; a chain extender N-vinyl        pyrrolidinone, LC E7, photoinitiator rose Bengal, and        coinitiator N-phenyl glycine. Surfactant octanoic acid was added        in certain variants.    -   Fontecchio et al., SID 00 Digest 774-776, 2000, the disclosure        of which is incorporated herein by reference, describes a UV        curable HPDLC for reflective display applications including a        multi-functional acrylate monomer, LC, a photoinitiator, a        coinitiators, and a chain terminator.    -   Y. H. Cho, et al., Polymer International, 48, 1085-1090, 1999,        the disclosure of which is incorporated herein by reference,        discloses HPDLC recipes including acrylates.    -   Karasawa et al., Japanese Journal of Applied Physics, Vol. 36,        6388-6392, 1997, the disclosure of which is incorporated herein        by reference, describes acrylates of various functional orders.    -   T. J. Bunning et al., Polymer Science: Part B: Polymer Physics,        Vol. 35, 2825-2833, 1997, the disclosure of which is        incorporated herein by reference, also describes multifunctional        acrylate monomers.    -   G. S. Iannacchione et al., Europhysics Letters Vol. 36 (6).        425-430, 1996, the disclosure of which is incorporated herein by        reference, describes a PDLC mixture including a penta-acrylate        monomer, LC, chain extender, coinitiators, and photoinitiator.

Acrylates offer the benefits of fast kinetics, good mixing with othermaterials, and compatibility with film forming processes. Sinceacrylates are cross-linked, they tend to be mechanically robust andflexible. For example, urethane acrylates of functionality 2 (di) and 3(tri) have been used extensively for HPDLC technology. Higherfunctionality materials such as penta and hex functional stems have alsobeen used.

Fabrication Methods for Multilayer Waveguide Stacks

The fabrication of multilayer optical devices in accordance with variousembodiments of the invention can include a variety of differenttechniques. Methods for recording optical structures into layers ofoptical recording material can be implemented for waveguide stacks withoverlapping layers. Such recording methods can include fabricating astack of optical structures in which a first optical recording materiallayer deposited on a substrate is exposed to form a first opticalstructure, which can be temporarily erased so that a second opticalstructure can be recorded into a second material layer deposited ontothe first layer using optical recording beams traversing the firstlayer. Although the recording methods are discussed primarily withregards to waveguide stacks with two overlapping layers, the basicprinciple can be applied to waveguide stacks with more than twooverlapping layers. Additionally, this principle can be applied tofabricate a variety of different waveguide stack configurations.

The basic principle of a method for recording a stack of two gratings inaccordance with various embodiments of the invention is conceptuallyillustrated in FIGS. 2A-2F. To simplify the description, the substratesor cells supporting the grating layer are not illustrated in FIGS.2A-2F. As discussed in the sections above, the gratings can be supportedby transparent substrates or encapsulated inside cells made fromtransparent substrates. FIG. 2A shows the first step 200A in which afirst layer 202 of optical recording material is provided. The recordingmaterial can include material systems capable of supporting opticalstructures that can be erased by a stimulus. Any of a variety ofdifferent types of optical recording material systems, such as but notlimited to the material systems described in the sections above, can beutilized. In many embodiments, the optical recording material includes amixture of liquid crystal and polymer. In some embodiments, the opticalrecording material can further include a photosensitive dye, aphotoinitiator, a surfactant, a multi-function monomer, and/ornanoparticles.

FIG. 2B shows a second step 200B in which an optical exposure process204 is applied to the first layer 202 to form a first optical structure206. In embodiments in which the optical structure is a holographicgrating, the exposure process can utilize a crossed-beam holographicrecording apparatus. In a number of embodiments, the optical recordingprocess uses beams provided by a holographic master, which may be aBragg hologram recorded in a photopolymer or an amplitude grating. Insome embodiments, the exposure process utilizes a single recording beamin conjunction with a master grating to form an interferential exposurebeam. In many embodiments, a contact copying apparatus using a mastergrating or hologram is used. In several embodiments, the opticalrecording process uses an apparatus for traversing recording beams witha predefined beam cross section along a predefined path across theoptical layer. As can readily be appreciated, optical structures can berecorded using a variety of exposure processes. In addition to theprocesses described, other industrial processes and apparatusescurrently used in the field to fabricate holograms can be used.

Turning now to FIG. 2C, a third step 200C is illustrated in which thefirst optical structure 206 is temporarily erased by an externalstimulus to produce a cleared first layer 208. External stimulus/stimulican include optical, thermal, chemical, mechanical, electrical, and/ormagnetic stimuli. In many embodiments, the external stimulus is appliedat a strength below a predefined threshold to produce optical noisebelow a predefined level. The specific predefined threshold can dependon the type of material used to form the optical structure. In someembodiments, a sacrificial alignment layer applied to the first opticalstructure can be used to temporarily erase the first optical structure.In some embodiments, the strength of the external stimulus applied tothe first optical structure is controlled to reduced optical noise inthe optical device during normal operation. In many embodiments, theoptical recording material further includes an additive for facilitatingthe process of erasing the optical structure, which can include any ofthe methods described above.

FIG. 2D shows a fourth step 200D in which a second layer 210 of opticalrecording material is brought into overlap with the exposed first layer202. In many embodiments, the optical recording materials in both layersare identical. In some embodiments, the first and second layers arefabricated using optical recording materials formulated to be recordablewith different spectral and/or angular bandwidths. Such materials can beoptimized for different ranges of spectral and/or angular bandwidths.

FIG. 2E shows a fifth step 200E in which an optical exposure process isapplied through the cleared first layer 202 and to the second layer 210to form a second optical structure 212. In many embodiments, during theexposure process, at least one light beam traverses through the firstlayer to record the second optical structure 212 in the second layer210.

Finally, FIG. 2F shows a final step 200F in which the first opticalstructure 206 has been restored to its recorded state. As shown, theresulting device includes two layers with optical structures 206, 212that are overlaid.

The clearing and restoration of a recorded layer described in theprocess above can be achieved using many different methods. In manyembodiments, the first layer is cleared by applying a stimuluscontinuously during the recording of the second layer. In otherembodiments, the stimulus is initially applied, and the grating in thecleared layer can naturally revert to its recorded state over atimescale that allows for the recording of the second grating. In otherembodiments, the layer stays cleared after application of an externalstimulus and reverts in response to another external stimulus. Inseveral embodiments, the restoration of the first optical structure toits recorded state can be carried out using an alignment layer or anexternal stimulus. An external stimulus used for such restoration can beany of a variety of different stimuli, including but not limited to thestimulus/stimuli used to clear the optical structure. Depending on thecomposition material of the optical structure and layer to be cleared,the clearing process can vary. In embodiments utilizing LC materials,the clearing process can be based on changing the order parameter of theliquid crystals. FIGS. 3A and 3B conceptually illustrate examples of anordered liquid crystal phase and a disordered liquid crystal phase,respectively. As discussed above, changing the order parameter of theliquid crystals can be achieved in various ways, including but notlimited to applying an external stimulus such as but not limited to anelectrical stimulus.

Multi-layer waveguide stacks can be fabricated using a variety ofdifferent methods. Additionally, multi-layer waveguide stacks can beconstructed with different materials in many different ways. In someembodiments, the waveguide stack includes at least two layers of exposedoptical recording material having optical structures. In furtherembodiments, the two layers of exposed optical recording material areseparated by a substrate. As can readily be appreciated, the specificmethod implemented can depend on the construction of the waveguidestack. FIGS. 4-7 conceptually illustrate several processes formanufacturing different types of waveguide stacks in accordance withvarious embodiments of the invention.

FIG. 4 conceptually illustrates a flow chart of a process 400 forfabricating a multi-waveguide layer stack in accordance with anembodiment of the invention, similar to the process conceptuallyillustrated in FIGS. 2A-2F. In the illustrative embodiment of FIG. 4,the method includes fabricating a waveguide stack in which a firstoptical recording material layer deposited on a substrate is exposed toform a first optical structure, which is temporarily erased so that asecond material layer deposited onto the first layer can be exposedusing recording beams traversing the first layer.

Turning now to the specifics of FIG. 4, the process 400 can includeproviding (402) a first optical substrate. Such optical substrates canvary in form and material, such as but not limited to glass andplastics. In many embodiments, the substrates are planar glass plates.In some embodiments, the substrates are curved. A first layer of opticalrecording material can then be deposited (404) onto the first opticalsubstrate. Material systems capable of being utilized in accordance withvarious embodiments of the invention can include any material systems inwhich optical structures can be recorded, such as but not limited toHPDLC material systems. Further examples and variations of such materialsystems are described in the sections above. Once the optical recordingmaterial is deposited, an optical exposure process can be applied (406)to the first layer of optical recording material to form a first opticalstructure. Optical exposure processes can include conventionaltechniques used within the field. In many embodiments, a crossed-beamholographic recording apparatus is utilized for the optical exposureprocess. In some embodiments, a contact copying apparatus using a mastergrating or hologram is utilized. In several embodiments, an apparatusfor traversing light with a predefined beam cross section is utilized.As can readily be appreciated, any optical exposure process can be used,the specific process of which can depend on the specific requirements ofa given application—i.e., different types of optical recording materialscan have different preferred optical exposure processes. Once the firstoptical structure is formed in the first layer, the first opticalstructure can be temporarily erased (408). Erasing the opticalstructure, or clearing the layer, can be achieved using any of themethods described above, such as but not limited to applying a stimulusto the first layer. A second layer of an optical recording material canbe deposited (410) onto the first layer. In several embodiments, theoptical recording material of the second layer is identical to theoptical recording material of the first layer. In other embodiments, theoptical recording materials of the two layers are different. An opticalexposure process can be applied (412) to the second layer to form asecond optical structure. Exposure processes can include any of theprocesses utilized for the exposing the first layer. In manyembodiments, the exposure process includes traversing at least one lightbeam through the first layer to record the second optical structure inthe second layer. Once the cleared first layer is restored, theresulting device is a two-layer device with each layer having at leastone optical structure.

FIG. 5 is a flow chart conceptually illustrating a process 500 forfabricating a multi-waveguide layer stack having two grating layersseparated by a substrate in accordance with an embodiment of theinvention. As shown, the process 500 can include steps that are similarto the method of FIG. 4. As such, it is to be understood that variousways of performing the steps of the process illustrated in FIG. 4 can beapplied similarly to the process 500 of FIG. 5. Referring to FIG. 5, theprocess 500 can include providing (502) a first optical substrate.Optical substrates can include plates made of transparent materials,such as but not limited to glass and plastics. Additionally, thesubstrates can be curved or planar. A first layer of optical recordingmaterial can be deposited (504) onto the first optical substrate.Various optical recording materials, such as but not limited to HPDLCmaterial systems, can be utilized. A first cover layer can optionally beapplied (506) to the first layer. In many embodiments, a cover layer isapplied as a substrate. In such cases, the cover layer can include anadhesive layer protected by a peel-off film. In some embodiments, theadhesive layer can be applied to the grating. In several embodiments,the cover layer is a protective layer deposited. Deposition of the coverlayer can employ various techniques, such as but not limited to the useof an inkjet coater and other coating processes. Various types of coverlayers can also be utilized. In some embodiments, the cover layer is aflexible glass foil, such as but not limited to Corning® Willow® Glass,which is available in various thicknesses including 100 and 200micrometers. As can readily be appreciated, the specific type of coverlayer utilized can depend on the given application. For example, inlarger displays, thicker cover glass can be utilized. One exampleinclude HUD applications, which can include the use of a 10 mm coverlayer. Other materials for cover layers include but are not limited toTAC (cellulose triacetate), TPU (thermoplastic polyurethane), and PET(polyurethane). In several embodiments, the cover layer is a multilayerstructure using different materials for greater robustness or foroptimizing optical properties, such as but not limited to transmissionand/or polarization response. In some applications, the use of a glassor plastic substrate can be eliminated by overcoating the grating layerwith a monomeric barrier film that seals the material. In manyembodiments, the barrier film is UV curable. In several embodiments, thecoating is carried out in a Nitrogen-rich atmosphere. One relevantinkjet coating process, known as the YIELDjet® FLEX system, has beendeveloped by Kateeva Inc. (CA) for OLED display large volumefabrication.

Referring back to FIG. 4, the process 500 can include applying (508) anoptical exposure process to the first layer to form a first opticalstructure. Optical exposure processes can include various holographicrecording techniques, such as but not limited to single or two-beaminterferential processes. As can readily be appreciated, any opticalexposure process can be used and can depend on the specific requirementsof a given application. The formed first optical structure can betemporarily erased (510). Erasing the optical structure, or clearing thelayer, can be achieved using any of the methods described above, such asbut not limited to applying a stimulus to the first layer. A secondsubstrate can be provided (512). A second layer of an optical recordingmaterial can be deposited (514) onto the second substrate. In severalembodiments, the optical recording material of the second layer isidentical to the optical recording material of the first layer. In otherembodiments, the optical recording materials of the two layers aredifferent. A second cover layer (516) can optionally be applied to thesecond layer. In many embodiments, the second cover layer is made of amaterial identical to the first cover layer. The first and secondsubstrates can be overlaid (518) with each other. In severalembodiments, the second substrate is laterally or rotationally displacedrelative to the first substrate. An optical exposure process can beapplied (520) to the second layer to form a second optical structure.Exposure processes can include any of the processes utilized for theexposing the first layer. In many embodiments, the exposure processincludes traversing at least one light beam through the first layer torecord the second optical structure in the second layer.

FIG. 6 is a flow chart conceptually illustrating a process 600 forfabricating a multi-waveguide layer stack in which each grating layer isencapsulated within a cell in accordance with an embodiment of theinvention. Referring to FIG. 6, the process 600 can include providing(602) a first optical substrate and a second optical substrate. Any typeof optical substrates, such as those described in the sections above,can be utilized. In many embodiments, the optical substrates aretransparent. A first cell can be formed (604) from the first and secondsubstrates. The first cell can be filled (606) with a first opticalrecording material. Various optical recording materials, such as but notlimited to HPDLC material systems, can be utilized. As can readily beappreciated, the formation of a cell containing optical recordingmaterial can be achieved in many different ways. In some embodiments,the cell is formed by depositing the optical recording material onto afirst substrate. The deposition technique can include but are notlimited to inkjet printing, spin-coating, and various additivemanufacturing depositions techniques. A second substrate can be placedon top of the deposited material and the various layers can be laminatedto form a cell. Referring back to FIG. 6, the process 600 can includeapplying (608) an optical exposure process to the first cell to form afirst optical structure. As can readily be appreciated, any opticalexposure process can be used and can depend on the specific requirementsof a given application. The first optical structure can be temporarilyerased (610). Erasing the optical structure, or clearing the layer, canbe achieved using any of the methods described above, such as but notlimited to applying a stimulus to the first layer. A third opticalsubstrate and a fourth optical substrate can be provided (612). A secondcell can be formed (614) from the third and fourth substrates. Thesecond cell can be filled (616) with an optical recording material. Inseveral embodiments, the second cell is filled with a type of opticalrecording material that is identical to the optical recording materialin the first cell. The first and second cell can be overlaid (618) witheach other. In several embodiments, the second cell is laterally orrotationally displaced relative to the first cell. An optical exposureprocess can be applied (620) to the optical recording material of thesecond cell to form a second optical structure. Various optical exposureprocesses, including but not limited to the exposure processes for thefirst cell, can be utilized.

Although FIGS. 2A-2F and 4-6 conceptually illustrate specific processesfor fabricating optical structures in a stack of two layers of recordingmaterial, it is to be understood that the basic principle can be appliedto many different fabrication processes and variations of such inaccordance with various embodiments of the invention. For example, theprocesses described above can be extended to allow for the recording ofstacks with more than two layers. In many embodiments, the processesdescribed can be implemented iteratively to record optical structures ina stack of more than two layers of recording material. In suchembodiments, a third layer of optical recording material can bedeposited and an optical exposure process can be applied to the thirdlayer to form an optical structure using a light beam that traversesthrough the erased first and second layers. In embodiments withprocesses for fabricating a waveguide stack with more than two layers,the methods for temporarily clearing the optical structures can beapplied to more than one layer simultaneously. In some embodiments,layers and substrates that provide a waveguide layer can be separatedfrom other waveguide layers by air gaps or layers of low refractiveindex material. In several embodiments, nanoporous material can be usedto separate the waveguide layers.

As described above, various steps in the processes conceptuallyillustrated in FIGS. 5 and 6 can be performed similarly to theircounterparts in the process conceptually illustrated in FIG. 4.Additionally, some steps can be performed independent of previous steps.Such steps can be performed concurrently or sequentially with othersteps. For example, in the process of FIG. 6, the second cell can beformed concurrently with, subsequent to, or prior to the formation offirst cell. In some embodiments, the order of some of the steps used inthe above processes can be changed.

The above described processes can further include at least one of thesteps of: applying electrodes to substrate surfaces for switchingoptical structures; providing an air gap in the stack of layers;applying a layer of low refractive index material; applying apolarization control layer; and/or applying a liquid crystal alignmentlayer. In such embodiments where a liquid crystal alignment layer isapplied, the liquid crystal alignment layer can be a liquid crystalpolymer or a linearly photopolymerizable (“LPP”) material. Inembodiments where a polarization layer is applied, the polarizationcontrol layer can be a half wave plate or a quarter waveplate.

It is to be understood that the various components, such as opticalsubstrates and optical recording materials, utilized in the processescan differ from application to application. Even among a singleapplication, different materials can be used. For example, in a givenapplication, optical substrates utilized can include plates made of atransparent material, such as glass or plastic. The plastic substratescan be fabricated in various ways, such as but not limited to using thematerials and processes disclosed in PCT Application No.:PCT/GB2012/000680, entitled IMPROVEMENTS TO HOLOGRAPHIC POLYMERDISPERSED LIQUID CRYSTAL MATERIALS AND DEVICES, the disclosure of whichis hereby incorporated by reference in its entirety. In manyembodiments, at least one of the substrates can be curved and fabricatedfrom various materials, such as but not limited to plastic and variousflexible materials. In some embodiments, the optical layer is formedinto a wedge by tilting one of the substrates. In several embodiments, awedged optical layer is formed by controlling the layer thickness in acoating process. In various embodiments, substrates of similar materialsare used. In other embodiments, different substrate materials can beused in the same application.

The same principle discussed above can be applied to the opticalrecording materials. Various optical recording materials, such as butnot limited to HPDLC mixtures, can be used in the processes describedabove. In several embodiments, the layers having optical structures canbe formed from the same type of optical recording material. In otherembodiments, each layer is formed from an optical recording materialthat is formularized for a different application that can differ fromthe formulation of the optical recording material in a different layer.In many embodiments, the optical recording material can be a low hazematerial, such as those described in U.S. patent application Ser. No.16/242,943, entitled LOW HAZE LIQUID CRYSTAL MATERIALS, the disclosureof which is hereby incorporated by reference in its entirety. In someembodiments, the optical recording material can be one optimized forrecording holographic gratings with high sensitivity to S and Ppolarized light, such as those described in U.S. patent application Ser.No. 16/242,954, entitled LIQUID CRYSTAL MATERIALS AND FORMULATIONS, thedisclosure of which is hereby incorporated by reference in its entirety.

Many embodiments in accordance with the invention can be applied invarious mass production processes. In some embodiments, fabricationprocesses, such as those described above, are implemented within aroll-to-roll fabrication process. In several embodiments, the processescan be used in the manufacturing of an environmentally isolatedwaveguide display according to the embodiments and teachings of U.S.patent application Ser. No. 15/543,016, entitled ENVIRONMENTALLYISOLATED WAVEGUIDE DISPLAY. The disclosure of U.S. patent applicationSer. No. 15/543,016 is hereby incorporated by reference in its entiretyfor all purposes. In a number of embodiments, the fabrication processescan be applied in the manufacture of a waveguide integrated within awindow as disclosed in the above reference.

In some embodiments, SBGs are recorded in a uniform modulation material,such as polymer liquid crystal polymer slices (“POLICRYPS”) or polymerliquid crystal polymer holograms electrically manageable (“POLIPHEM”)mixtures having a matrix of solid liquid crystals dispersed in a liquidpolymer. The SBGs can be switching or non-switching in nature. In itsnon-switching form, an SBG has the advantage over conventionalholographic photopolymer materials of providing high refractive indexmodulation due to its liquid crystal component. Exemplary uniformmodulation liquid crystal-polymer material systems, characterized byhigh refractive index modulation (and hence high diffraction efficiency)and low scatter, are disclosed in United State Patent ApplicationPublication No.: 2007/0019152 by Caputo et al. and PCT Application No.:PCT/EP2005/006950 by Stumpe et al., the disclosures of which areincorporated herein by reference in their entireties for all purposes.

In many embodiments, the gratings are recorded in a reverse mode HPDLC,which differs from conventional HPDLC in that the grating is passivewhen no electric field is applied and becomes diffractive in thepresence of an electric field. The reverse mode HPDLC can be based onany recipes and processes, such as those described in PCT ApplicationNo.: PCT/GB2012/000680, entitled IMPROVEMENTS TO HOLOGRAPHIC POLYMERDISPERSED LIQUID CRYSTAL MATERIALS AND DEVICES, the disclosure of whichis hereby incorporated by reference in its entirety. The grating can berecorded in any of the above material systems and used in a passive(non-switching) mode. The fabrication process can be identical to thatused for switched gratings but with the electrode coating stage beingomitted.

Although specific fabrication processes are discussed above, manydifferent processes can be implemented in accordance with many differentembodiments of the invention. It is therefore to be understood that thepresent invention can be practiced in ways other than specificallydescribed, without departing from the scope and spirit of the presentinvention. Thus, embodiments of the present invention should beconsidered in all respects as illustrative and not restrictive.Accordingly, the scope of the invention should be determined not by theembodiments illustrated, but by the appended claims and theirequivalents. Although specific embodiments have been described in detailin this disclosure, many modifications are possible (for example,variations in sizes, dimensions, structures, shapes and proportions ofthe various elements, values of parameters, mounting arrangements, useof materials, colors, orientations, etc.). For example, the position ofelements may be reversed or otherwise varied and the nature or number ofdiscrete elements or positions may be altered or varied. Accordingly,all such modifications are intended to be included within the scope ofthe present disclosure. The order or sequence of any process or methodsteps may be varied or re-sequenced according to alternativeembodiments. Other substitutions, modifications, changes, and omissionsmay be made in the design, operating conditions and arrangement of theexemplary embodiments without departing from the scope of the presentdisclosure.

Doctrine of Equivalents

While the above description contains many specific embodiments of theinvention, these should not be construed as limitations on the scope ofthe invention, but rather as an example of one embodiment thereof. It istherefore to be understood that the present invention may be practicedin ways other than specifically described, without departing from thescope and spirit of the present invention. Thus, embodiments of thepresent invention should be considered in all respects as illustrativeand not restrictive. Accordingly, the scope of the invention should bedetermined not by the embodiments illustrated, but by the appendedclaims and their equivalents.

What is claimed is:
 1. A method of fabricating an optical element, themethod comprising: providing a first optical substrate; depositing afirst layer of a first optical recording material onto the first opticalsubstrate; applying an optical exposure process to the first layer toform a first optical structure; temporarily erasing the first opticalstructure; depositing a second layer of a second optical recordingmaterial; and applying an optical exposure process to the second layerto form a second optical structure, wherein the optical exposure processcomprises using at least one light beam traversing the first layer. 2.The method of claim 1, further comprising: providing a second opticalsubstrate, wherein the second layer is deposited onto the second opticalsubstrate; and overlapping the second optical substrate with the firstoptical substrate.
 3. The method of claim 2, wherein the second opticalsubstrate is laterally or rotationally displaced relative to the firstoptical substrate.
 4. The method of claim 1, further comprising:applying a first cover layer to the first layer; and applying a secondcover layer to the second layer.
 5. The method of claim 1, wherein theat least one light beam is provided by an apparatus selected from thegroup consisting of: a crossed-beam holographic recording apparatus; acontact copying apparatus using a master grating or hologram; and anapparatus for traversing light with a predefined beam cross section. 6.The method of claim 1, wherein the first optical structure istemporarily erased by applying an external stimulus.
 7. The method ofclaim 6, wherein the external stimulus comprises a stimulus selectedfrom the group consisting of: an optical stimulus; a thermal stimulus; achemical stimulus; a mechanical stimulus; an electrical stimulus; and amagnetic stimulus.
 8. The method of claim 6, wherein the externalstimulus is applied at a strength below a predefined threshold toproduce optical noise below a predefined level.
 9. The method of claim1, further comprising: temporarily erasing the second optical structure;depositing a third layer of a third optical recording material; applyingan optical exposure process to the third layer to form a third opticalstructure using at least one light beam traversing the first layer andthe second layer.
 10. The method of claim 1, wherein at least one of thefirst and second optical structures modifies at least one of: phase;amplitude; and wavefront of incident light.
 11. The method of claim 1,wherein the first optical recording material and the second opticalrecording material comprise different material formulations.
 12. Themethod of claim 1, wherein the first optical recording materialcomprises a mixture of liquid crystal and polymer; and wherein the firstoptical structure comprises at least one grating.
 13. The method ofclaim 12, wherein the first optical recording material further comprisesat least one of: a LPP; a dye; a photoinitiator; a surfactant; amulti-function monomer; and nanoparticles.
 14. The method of claim 12,wherein temporarily erasing the first optical structure compriseschanging the order parameter of the liquid crystal.
 15. The method ofclaim 1, wherein the first optical recording material comprises a liquidcrystal, polymer, and an additive for temporarily erasing the firstoptical structure.
 16. The method of claim 1, wherein the first opticalrecording material is deposited onto the first optical substrate usingspin coating or inkjet printing.
 17. The method of claim 1, wherein thefirst optical substrate is curved.
 18. The method of claim 1, furthercomprising at least one of the steps of: forming an air gap; applying alayer of low refractive index material; applying a polarization controllayer; and applying a liquid crystal alignment layer.
 19. The method ofclaim 1, forming part of a roll-to-roll fabrication process.
 20. Amethod of fabricating an optical element, the method comprising:providing first and second optical substrates; forming a first cell fromthe first and second substrates; filling the first cell with a firstoptical recording material; applying an optical exposure process to thefirst cell to form a first optical structure; temporarily erasing thefirst optical structure; providing third and fourth optical substrates;forming a second cell from the third and fourth substrates; filling thesecond cell with a second optical recording material; overlapping thefirst and second cells; and applying an optical exposure process to thesecond layer to form a second optical structure, wherein the opticalexposure process comprises using at least one light beam traversing thefirst layer.